This invention relates to a flame retardant for halogen-containing vinyl resins in which antimony pentoxide is contained as a main component, more specifically to a flame retardant for halogen-containing vinyl resins in which antimony pentoxide (Sb.sub.2 O.sub.5) is contained as a main component and which does not impair the transparency or the thermal stability of vinyl chloride resins.
The halogen-containing vinyl resin itself has excellent flame retardancy, but a so-called flexible vinyl chloride resin plasticized with a plasticizer such as dioctyl phthalate and a halogen-containing vinyl resin blended with a combustible resin are easily burnt. With regard to flexible vinyl chloride products such as leathers, hood cloths, wire-covering materials, films and sheets which are widely utilized as interior materials for automobiles, electrical and electronic parts, and architectural materials, or rigid vinyl chloride products blended with a reinforcing material against shock, it is especially essential to give flame resistance.
In order to provide these resins with the flame resistance, a variety of flame retardants have been used. Examples of the flame retardants which are used include inorganic materials such as antimony trioxide (Sb.sub.2 O.sub.3), sodium antimonate, aluminum hydroxide, zirconium oxide, zinc borate and borax as well as organic materials such as tricresyl phosphate, trichloroethyl phosphate and chlorinated paraffins. Further, organic flame retardants containing bromine are also employed in rare cases. These flame retardants mentioned above have some advantages and disadvantages from the viewpoints of flame-resisting effect, transparency, thermal stability, weather resistance, cold resistance, bleed/bloom resistance and the like.
Antimony trioxide displays an extremely great opacifying effect, since it generally has a particle diameter of 0.5 to 10.mu. or so, a large refractive index and a small light transmittance. Therefore, the transparency which is the greatest feature of the halogen-containing vinyl resin will be completely lost, if the above-mentioned antimony trioxide is mixed with the resin. Further, when a pigment is added to the vinyl resin containing the antimony trioxide with the intention of coloring, a great deal of the pigment is required because of its opacifying effect, and kinds of obtainable colors are disadvantageously limited. In order to overcome these drawbacks, it has been suggested to excessively lessen the particle diameter of the antimony trioxide (Japanese Patent Publication No. 7170/1971) and to conversely increase the particle diameter by the single crystallization. Both of them contemplate improving the light transmittance. In the case of the former above, the transparency can remarkably be enhanced, but if the pigment is added, the opacifying function will be heightened, the thermal stability also will become poor, and a manufacturing cost will rise. In the case of the latter above, the sufficient transparency cannot be obtained.
Antimony pentoxide is a flame retardant which is applicable to the halogen-containing vinyl resin which does not cause transparency of the resin to be lost (J. Coated Fabric, Vol. 11, 1982, p. 137). This flame retardant is antimony pentoxide tertrahydrate which is prepared by bringing an antimony pentoxide sol (its particle diamerter ranges from 20 to 100 m.mu.) into a powdery form with the aid of spray drying or the like. As for this antimony pentoxide tetrahydrate, a refractive index is small and primary particles are extremely fine. Therefore, the antimony pentoxide tetrahydrate has a great light transmittance, which fact indicates that the transparency of the resin is very good. This antimony pentoxide, however, has a drawback of worsening the thermal stability of the resin (see Comparative Examples set forth hereinafter). For the purpose of eliminating this drawback, it has been suggested to make use of a Ba-Cd-Zn stabilizer or a tin mercaptide stabilizer (J. Coated Fabric, Vol. 11, 1982, p. 137). However, these stabilizers cannot be practically used, because of the formation of harmful cadmium and the bleed/bloom in the case of the Ba-Cd-Zn stabilizer, and because of a bad weather resistance in the case of the tin mercaptide stabilizer. Sodium antimonate, when having a large particle diameter, is poor in flame-resisting effect, but it has been reported that when ground up to a level of 0.1 to 8.mu. and neutralized with an acid, the sodium antimonate exhibits good transparency, flame retardancy and thermal stability (Japanese Patent Publication No. 36863/1972, which corresponds to U.S. Pat. No. 3,897,389). However, it is clear that the sufficient transparency of the resin cannot be obtained in the range of such a primary particle diameter, and that since this neutralization is partially carried out and thus most of the sodium antimonate remains as it is, sufficient flame retardancy cannot be obtained.
Inorganic materials such as aluminum hydroxide, zirconium oxide, zinc borate and borax are poorer in the flame-resisting effect than antimony trioxide and antimony pentoxide, and have the behavior of losing the transparency of the resin like the antimony trioxide. A phosphorous plasticizer such as tricresyl phosphate and a chlorinated paraffin which have no behavior of losing the transparency but which have a plasticizisng action are poorer in the flame-resisting effect than the antimony trioxide, therefore they must be added in greater amounts. As a result, the bleed will tend to occur and the cold resistance will deteriorate. In consequence, the conventional products and techniques are incapable of imparting flame retardancy to the halogen-containing vinyl resin in an economical manner without imparing any transparency which is the greatest feature thereof, and at the same time satisfying physical properties such as the thermal stability.